Protected hot wire ignition system



Jan. 6, 1970 R. 1.. PERL.

PROTECTED HOT WIRE IGNITION SYSTEM Original Filed Oct. 18, 1967ooeouanaooooouoo INVENTOR RICHARD L. PERL ATTORNEYS United States Patent3,488,133 PROTECTED HOT WIRE IGNITION SYSTEM Richard L. Perl, Mansfield,Ohio, assignor to The Tappan Company, Mansfield, Ohio, a corporation ofOhio Continuation of application Ser. No. 676,149, Oct. 18, 1967. Thisapplication Jan. 9, 1969, Ser. No. 791,872

Int. Cl. F23n /00 US. Cl. 43166 9 Claims ABSTRACT OF THE DISCLOSURE Anignition system for a gas burner using a high temperature wire ignitor,preferably of molybdenum disillcide, having two coils of differentdiameter and resistance value, the two in electrical series. The smallerdiameter coil is the ignition element and the other a ballast .coilarranged next to the burner and between the same and the ignition coil.When the burner is turned on, current flows through the coils and acurrent relay in series which opens a solenoid valve in the gas line tothe burner; the ignition current fiow heats the ignition coil to ignitethe raw gas from the burner while the ballast is relatively cool. Theballast coil closer to the flame after ignition is heated by the same toincrease its resistance and cut down the current flow in the ignitioncircuit to prolong the coil life.

This application is a continuation of application Ser. No. 676,149,filed Oct. 18, 1967, now abandoned.

DISCLOSURE This invention relates to an electric ignition system for gasburners, such as employed in domestic ovens and cook tops, in which theigniting element is a wire in a continuous direct resistance heatingcircuit.

This type of ignition has generally been avoided in such appliancesbecause of fairly rapid deterioration and failure of the elements orcoils which have been tested, primarily due to overheating when placedat good ignition locations, and the difficulty has been greatlyemphasized in the current development of the so-called selfcleaning gasovens in which a temperature of approximately 1000 F. may be reached ina cleaning cycle. Resort has thus been had to spark ignitor systems forsuch ovens, which are relatively more complicated, expensive and subjectto malfunction and,v according at least to some designs, arecontinuously throughout the whole period of the burner operation.

There has been some improvement in hot-wire ignitors in the sense of newmaterials with superior operating characteristics, with the molybdenumdisilicide wire trademarked Kanthal notable in this respect since it hasan operating temperature of about 3000 F. and good life. However, thiswire is very brittle which makes it difficult to work in the forming ofan ignitor from the same, and the problem of overheating would stillexist in the normal relation to the burner within the oven.

It is a primary object of the present invention, accordingly, to providesuch a hot-wire ignition system for a gas oven burner in which theignition element is protected from the usual overheating due to theburner operation and the life of the element thereby prolonged.

Another object is to provide such a system in which the flow of thegaseous mixture to the burner is dependent upon integrity of theignition element and, in the continuing sense, upon combustion of themixture being initiated and maintained.

It is also an object of the invention to provide an improvedconfiguration or mechanical design for such a hot-wire ignition element.

3,488,133 Patented Jan. 6, 1970 ice Other objects and advantages of thepresent invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawing setting forth in detail a certain illustrativeembodiment of the invention, this being indicative, however, of but oneof the various ways in which the principle of the invention may beemployed.

In said annexed drawing:

FIG. 1 is a schematic diagram of a gas oven ignition system inaccordance with the present invention;

FIG. 2 is an elevation of a hot-wire ignitor used in this system, on anenlarged scale;

FIG. 3 is a side elevation of the ignitor;

FIG. 4 is a simplified and fragmented plan view of a fixture employed informing the hot-wire element; and

FIG. 5 is a cross-sectional view of the FIG. 4 assembly as shown thereinby the line 5-5.

Referring now to the drawing in detail, the full system shownschematically in FIG. 1 is applied to th control of a cooking ovencomprising an upper burner 10 and a lower burner 11, with the specificforms of such burners being without particular significance in thepresent improvements. The oven structure itself can likewise be of anysuitable form and accordingly is not illustrated.

The operation of the two burners and hence the representative oven isbasically programmed by a temperature controller T which comprises inits simplest form a thermostatically actuated switch contactor 12 andfirst and second adjustable switch contacts 13 and 14, respectively, theadjustment of which is effected by manual setting of the controller. Thecontactor cooperates with fixed contacts 15 in a line from the energysource terminal 16 to the oven control circuit to be more fullydescribed for control of the energization of the latter in accordancewith the oven temperature. The contactor is shown as operated by athermostat the sensor 17 of which is suitably disposed within the oven;other specific temperature responsive devices could obviously beutilized and will in all cases provide for manual setting or adjustmentof the desired temperature in the usual manner.

The control circuit extends from the contacts 15 to a normally openmovable contact 18 of a current relay 19 the fixed contact 20 of whichis connected by a wire 21 to the first movable switch contact 13 of thetemperature controller. This contact 18 can selectively be moved toengage either a contact 22 from which a wire 23 extends to one side of afirst solenoid valve V1 or another stationary contact 24 connected bywire 25 to one side of a second solenoid valve V2. The other sides ofthe two solenoid valves are shown as having a common ground connection26.

The gaseous fuel mixture to the burners is delivered from a suitablesupply line 27 through a manual valve V to the fuel lines 28 and 29which extend respectively to the upper and lower burners. The solenoidvalves V1 and V2 are located in the branch lines 28 and 29 and,accordingly, the adjustment of the switch contact 13 in the setting ofthe temperature controller T conditions one or the other of the burnersfor operation, for example, with the upper burner to be used forbroiling and the lower burner selected for baking as is conventional.

The relay 19, which obviously must be energized to operate the selectedsolenoid valve V1 or V2, is in a separate circuit at a lower voltage,for example 6 volts, derived from a stepdown transformer 30. Thesecondary of this transformer is connected to one end of the relay coiland to upper and lower wire ignition devices desig nated generally byreference numerals 31 and 32. The

latter are respectively connected to stationary contacts 33 and 34associated with the second controller adjustable contact 14 which is, inturn, connected to the other end of the relay coil. The relay 19 istherefore in series with one or the other of the ignitors depending uponthe adjustment of the controller contact 14 in the programming of theoven operation.

The ignitors 31 and 32 are respectively disposed at ignition locationsfor the upper and lower burners, with each positioned in the path of rawgas issuing from a section of the associated burner when the mixture issupplied to the same and effective to ignite the mixture at such placeby being heated to a sufiiciently high temperature as a result of theflow of electric current through the same. The details of the preferredform for the ignitor will be further described below, but it will beappreciated from the schematic of the system that it is impossible tooperate either burner unless the associated ignitor is energized. Forexample, in the circuit condition shown in full lines in FIG. 1, thecontroller T has been set for operation of the lower or bake burnerlland, the setting also closing the thermostatic contactor 12, a bottomignition circuit is completed by closure of the contacts 14 and 34through relay coil 19 and ignitor 32 in series. If the latter is intact,the current flow through this circuit effects the desired heating of theignitor to the temperature for ignition and the relay contacts 18 and 20are closed to complete the circuit through the contacts 13 and 24 forenergization of the solenoid valve V2. Similarly, adjustment of thecontroller T to close contacts 13 and 22 readies the solenoid valve V1,the upper ignitor circuit is energized through closure of the contacts14, 33 and the relay 19 is again closed for opening actuation of thevalve V1. In either case, a break in the ignitor element would precludeoperation of the relay 19 and thus protect against any flow of the fuelmixture to the selected burner.

Each ignitor device 31 and 32 comprises two serially connected coils,and these are preferably formed of the previously noted molybdenumdisilicide or equivalent wire. As shown more in detail in FIGS. 2 and 3,which can be taken to illustrate the upper ignition device 31 upon theunderstanding that the lower one is identical, the wire is formed into afirst coil 35 with a given crosssectional area or wire diameter and asecond coil 36 of significantly larger wire diameter. The first coil isthe actual ignition element, while the second is a ballast coil, andtheir respective resistance values are such that the current in theignition circuit causes the ignition coil 35 rapidly to heat toincandescence without comparable heat development in the series ballastcoil 36. For example, the ignition coil can be brought by such directresistance heating quickly to an igniting temperature in excess of 2000F., while the ballast coil temperature does not exceed half of thisvalue and can be as low as 500 F. or less. It is also significant thatthe ignitor is arranged relative to the burner so that the ballast coil36 is closest to the same and hence the flame, as indicated at F in FIG.3, while the burner is operating.

With this arrangement, when the raw gas first issues from the portadjacent the ignitor, it flows over the ballast coil 36 and initiallyhas a cooling effect on the same, which further serves to hold this coilrelatively cool until the burner ignites. The wire has of course apositive temperature coeflicient of resistance and, after the burnerignites, the ballast coil will be significantly heated by the flame withthe result that the current in the circuit is decreased. The ignitioncoil temperature drops by reason of the currentdrop and the useful lifeof the same is thereby extended, while higher and more positive ignitiontemperatures, for example on the order of 2400 F., can initially bereached.

It is preferred that the two coils 35 and 36 be formed from a singlelength of the wire, such as the noted molybdenum disilicide, and the deired variation in d ameter can be accomplished in a number of ways. Suchvariation can, for example, be realized by extruding the wire, pressing,rolling, or chemically etching at a variable rate. It is particularlydifficult to work this type of wire, especially to provide the coil formwhich is most eflicient for the intended application, but coiling can beaccomplished by closely controlled high temperature heating in themanner indicated in FIGS. 4 and 5. In these figures, a length of thewire W is shown as gripped in a holder 37 at one end and resting on thetop of a ceramic body 38 in which there is a gaseous fuel chamber 39with a top opening. A ceramic post 40 extends vertically through thechamber and projects above the top surface of the body to define arelatively narrow annular port for the chamber at which the mixture isburned to provide a sleeve of flame 41 closely about the projecting partof the post. The wire W is brought against the post 40 and heated over asmall section by the flame to a temperature of about 2800 F., which willpermit bending without fracture at the very localized heated area. Thewire is then manipulated to be progressively warped around the post withprogressive heating of the same in small increments and the coil orcoils thus formed.

The ignitor device 31 comprises an inner insulative body 42 throughwhich first and second terminal wires 43, 44 extend from upper loopedends to lower ends in sockets 45 and 46 where they are connected toinsulated supply wires 47, 48. The ends of the ignition-ballast coils35, 36 are attached to the looped terminal ends by high-temperaturesolder or welding, while a third wire loop 49 is provided irftheinsulation body 42 to support the mid-point of the coils as shown,without electrical connection.

, The insulation body 42 is received within an inner metal housing 50which includes an upper cap 51 serving as a shield for the coils. Thiscap has open ends with the edges at the top and sides at each end cutand bent inwardly to form tabs 52 to partially obscure and thus protectthe coils. The inner housing fits within an outer housing 53 and thelatter is shown as slotted at its sides for projection of spring arms 54provided in the inner housing 50. These outwardly biased arms areemployed for gripping of the device within a separate mounting, notshown, to hold the same at the selected ignition location adjacent theburner.

It will therefore be seen that high-temperature metals and alloys, whichare inherently brittle, difiicult to work, and; still subject to toorapid deterioration in excessive heat exposure can be applied to gasburner hot wire ignition by the disclosed improvements with the neededreliability and the desired economy particularly as compared toavailable spark ignition systems.

In the preferred form of the ignitor, the above coiling operation wouldof course be preceded by the working described as providing the twosections of varying diameter in a single length of the wire, but twoseparate lengths of such different diameter could if desired be employedrespectively for the ballast and ignition sections with the sangeoperative effect and control function. Where more than one ignitor isused as in the described oven system, they may be connected inelectrical series and both therefore operated if any burner is to beignited, with this modification of the circuit eliminating the lowvoltage switching in the previously disclosed parallel arrangement ofsuch devices. It will also be appreciated that in this case as well thedesired protection against operation due to a break in the ignitor orignitors is assured by the series relation to the relay controlling thesolenoid gas valves, with hazard as a result of shorting relativelyinconsequential in such low voltage circuitry.

I claim:

1. In combination with a burner to which a combustible gaseous mixtureis supplied and issues therefrom at a combustion outlet, an electricignition system therefor, comprising a high resistance ignitor wiresection, a ballast wire ction of lower resistance and having a posi ivet pe a ture coefficient of resistance connected in electrical seriescircuit relation to the ignitor section, means for mounting the ignitorwire section at an ignition location relative to the stream of the rawmixture issuing from the outlet when supplied to the burner and theballast section in exposure to the resulting burner flame for heatingthereby, and means for flowing electric current through the wiresections at a value so related to the diifering resistances thereof thatthe ignitor section heats rapidly by direct resistance heating to thetemperature at which the same ignites the raw mixture which the ballastsection is heated to an appreciably lesser degree, the exposure of theballast wire to heating by the burner flame after ignition substantiallyraising the temperature and resistance thereof to reduce the currentflow through the sections while the burner is operating.

2. The combination set forth in claim 1, wherein the ballast section islocated in the path of the raw mixture issuing from the burner outletbetween the latter and the ignitor wire section.

3. The combination set forth in claim 1, including control means forpreventing the supply of the mixture to the burner in the event of abreak in the circuit of the wire sections.

4. The combination set forth in claim 3, wherein the control meansincludes valve means in a mixture supply line to the burner and meansresponsive to current flow in the wire section circuit for opening saidvalve means.

5. The combination set forth in claim 1, wherein the wire sections aremade of a material having the characteristics of molybdenum disilicide.

6. An electric ignitor for a gas burner, comprising wire having discretesections of appreciably different electrical resistance values and apositive temperature coeflicient of resistance, such that current flowtherethrough produces relatively low direct heating of the section withthe lesser resistance in the interval required to heat the other sectiondirectly to a high temperature sufiicient to ignite the combustible gasmixture with which the burner is operated, and means for mounting thewire With said sections thereof proximate to each other for exposure ofboth to the gas mixture issuing from the burner and the flame resultingfrom ignition of the same.

7. An ignitor as set forth in claim 6, wherein the wire is made of amaterial having the characteristics of molybdenum disilicide.

8. An ignitor as set forth in claim 6, wherein the sections aresuccessive coils of the wire of different wire diameter.

9. An ignitor as set forth in claim 8, wherein the wire is made of amaterial having the characteristics of molybdenum disilicide.

References Cited UNITED STATES PATENTS 10/1944 Weber 43166 7/1965 Milleret al 43166 US. Cl. X.R.

